feat(notifications): persistent read cursor + delivery history

notification_cursors table stores one read-cursor per user per tenant;
GET /v1/activity returns unreadCount + readAt, POST /v1/activity/read
persists the cursor. Reminder dispatches now append reminder.sent /
reminder.failed system events so delivery history reaches the feed.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/backend/internal/api/server.go

@@ -1296,6 +1296,19 @@ func NewServer(cfg config.Config, pools *db.Pools) (*Server, error) {
 		c.JSON(http.StatusOK, response)
 	})
 
+	protected.POST("/activity/read", func(c *gin.Context) {
+		readAt, err := activityService.MarkAllRead(c.Request.Context(), auth.PrincipalFromContext(c))
+		if err != nil {
+			status := http.StatusInternalServerError
+			if errors.Is(err, activity.ErrTenantMembership) {
+				status = http.StatusNotFound
+			}
+			c.JSON(status, gin.H{"error": err.Error()})
+			return
+		}
+		c.JSON(http.StatusOK, gin.H{"readAt": readAt})
+	})
+
 	protected.GET("/metrics/staff", func(c *gin.Context) {
 		days, _ := strconv.Atoi(c.DefaultQuery("days", "30"))
 		response, err := bookingService.StaffMetrics(c.Request.Context(), auth.PrincipalFromContext(c), days)